1. Field of the Invention
The invention relates to a positioning device that is designed for, for example, a machine tool in which a ball screw is used, and that is able to more accurately compensate an error due to a temperature change.
2. Discussion of Background
There is a positioning device in which a ball screw is supported at respective end portions thereof by a reference holding member and a measurement holding member that are fixed on a base, and a movable body is positioned through the use of the ball screw. There is known a thermal displacement compensation method in which, in the above-described positioning device, the amount of rotation of the ball screw is adjusted on the basis of the amount of linear expansion of the base due to a change in the temperature of the base and the amount of linear expansion of the ball screw with respect to the measurement holding member, so that an error in the positioning amount is compensated (see Japanese Patent Application Publication No. 2010-284737 (JP-2010-284737 A)).
In the conventional positioning device described in Japanese Patent Application Publication No. 2010-284737 (JP-2010-284737 A), on the assumption that the temperatures of the reference holding member and the measurement holding member are equal to the temperature of the base, the temperature of the base is measured and only the amount of linear expansion of the base is obtained to compensate the positioning amount. Therefore, in the case where the temperatures of the reference holding member and the measurement holding member are different from the temperature of the base, an error is caused in the amount of linear expansion. Usually, heat is generated at ball screw holding portions due to rolling friction of a bearing. Therefore, the temperatures of the reference holding member and the measurement holding member are higher than the temperature of the base. Accordingly, with the conventional method, an error based on the amounts of linear expansion of the reference holding member and the measurement holding member is contained in the compensated positioning amount.